1. Field of the Invention
This invention relates to an apparatus and a method for estimating missing Doppler signals and spectra.
2. Description of Related Art
In ultrasound imaging systems receiving Doppler signals, there may be several different signals being acquired at once, such as B-mode signals, color Doppler signals, and nonimaging (audio/spectral) Doppler signals. Differing signals are often acquired in a scan line by scan line manner or in a block-interleaved fashion. Block-interleaved signal acquisition comprises periodically allocating a first block of time to acquisition of a first signal and a second block of time to acquisition of a second signal, thus providing a user with an illusion of simultaneous acquisition of multiple signals.
One problem which has arisen in the art is that block-interleaved signal acquisition introduces temporal gaps in the audio/spectral Doppler data. For example, while some other signal is being acquired, the audio/spectral Doppler signal is not being acquired, leaving a data gap. In general, it has been sought to estimate the missing signals, so as to provide a continuous data stream.
One method of the prior art has been to "stretch" the spectral data along the time axis, so that for example, 1 unit of time worth of data would supply 2 units of time worth of signal. This method is illustrated by FIG. 1. A Doppler spectral strip S is seen to vary in frequency content between frequencies f0 and f1, with time increasing from times t0 to t4. (Spectral Doppler signals are normally displayed in "velocity" units in mm/second, which is proportional to frequency shift.) If the signal S1 between times t1 and t2 were missing, i.e., there is a data gap between times t1 and t2, this prior art method would stretch the spectral strip S0, between times t0 and t1, to cover the period between times t0 and t2. Similarly, if the signal S3 between times t3 and t4 were missing, this method would stretch the spectral strip S2, acquired between times t2 and t3, to cover the period between times t2 and t4.
While this method of stretching spectral data along the time axis achieves the goal of providing a continuous data stream, it has the serious drawback of distorting the Doppler spectral strip. A first distortion is that the shape of the spectral strip can be significantly altered by stretching, to the point where there can be a complete mismatch between the spectral strip computed when gaps are present and the spectral strip computed when no gaps are present. This is seen by comparison of the Doppler spectral strip S of FIG. 1A between times t0 and t2, and the stretched Doppler spectral strip S0 of FIG. 1C between times t0 and t2. A second distortion is that the stretching operation can introduce discontinuities between Doppler segments. This is seen by noting the discontinuity at time t2 in FIG. 1C.
Another problem with the stretching method is that the method is only applicable to data gaps in spectral Doppler data, and is not generally applicable to data gaps in audio Doppler data.
Accordingly, it would be advantageous to provide a method and system for estimating either data or spectra generated from the data when there are data gaps in spectral Doppler data, which (1) do not severely distort the data (2) are not subject to the drawbacks of known methods, and (3) provide for presentation of either the data or the spectra of the data using audio/spectral Doppler presentation techniques.